This invention pertains to a method of phosphatization that is primarily employed as a lubricating pretreatment for cold plastic working of iron and steel.
In general, iron and steel are made into steel tubing, wire rods, and forged articles of manufacture from a raw material called a billet by hot processing and are then processed into a specified final shape and form by means of cold plastic working, such as wire drawing, tube reducing, forging, and cold pressing.
However, there is great mechanical resistance to shaping during cold plastic working of most commercial alloys of iron and steel, making the process difficult; therefore, the work pieces are usually first lubricated prior to processing. The lubricating method most often employed is to form a film such as a phosphate film on the surface of the material, then to coat this film with lubricating oil or a solid lubricant.
In the lubricating process, the formed film is used as the base for lubrication. For example, in wire drawing, the lubricant is drawn into the die, preventing the metals of the tool and the metal material from coming into contact with each other during the process (cf. Manual of Lubricants for Wire Drawing of Iron and Steel, edited by the Plastic Working Institute, Wire Drawing Technology Division, Subcommittee on Lubricants, 1994).
Dissolution of the metal is the driving force of film formation. For example, a treatment solution whose primary components are zinc ions and phosphate ions is used in the phosphatization treatment called the zinc phosphating system, and minute anode portions on the surface of the iron and steel are corroded by the etching effect of the phosphoric acid. The hydrogen ions in the treatment solution are reduced at minute cathode portions to form hydrogen gas; it is believed that this reaction increases the pH value of a thin zone adjacent to the interface between the treatment solution and the surface of the iron and steel, forming a phosphate film that is not highly soluble. Because the film is formed utilizing a dissolution reaction, the film adhesion is excellent, favoring superior coating adhesion and cold workability.
There are several types of such phosphate film, but those most widely used as the base for lubrication of iron and steel are zinc phosphate films with the approximate chemical formula Zn3(PO4)2.4H2O), zinc ferrous phosphate film with the approximate chemical formula Zn2Fe(PO4)2.4H2O, and zinc calcium phosphate film with the approximate chemical formula (Zn2Ca(PO4)2.2H2O) (cf. Manual of Lubricants for Wire Drawing of Iron and Steel, edited by the Plastic Working Institute, Wire Drawing Technology Division, Subcommittee on Lubricants, 1994, p. 24). Phosphate crystals with one of the above approximate chemical formulas rarely are the sole constituent of a phosphate film; instead these crystals coexist in a mixture in phosphate films formed under the usual conditions.
For example, zinc phosphate and zinc ferrous phosphate are obtained by treatment of a ferriferous substrate with a solution whose primary constituents are zinc ions and phosphate ions, but these crystal constituents are usually eutectic, and rarely crystallize alone. Similarly, zinc calcium phosphate is obtained by treatment with a solution whose primary constituents are zinc ions, calcium ions, and phosphate ions, but is eutectic with zinc phosphate and zinc ferrous phosphate.
Thus, phosphatization can be roughly divided into two types: (1) zinc-type phosphatization, in which the materials are treated with a solution whose primary constituents are zinc ions and phosphate ions; and (2) calcium-type phosphatization, in which the materials are treated with a solution whose primary constituents are zinc ions, calcium ions, and phosphate ions.
Zinc-type phosphatization and calcium-type phosphatization are used as the base for lubrication of iron and steel, the type to be used being determined by the purpose. Zinc-type phosphatization is suited for overall cold plastic working, and various lubricants are used after phosphatization. In particular, lubrication treatment in conjunction with a reactive soap lubricant imparts superior lubricity, and therefore is used in the most demanding processes, such as cold forging and cold pressing (cf. Manual of Lubricants for Wire Drawing of Iron and Steel, edited by the Plastic Working Institute, Wire Drawing Technology Division, Subcommittee on Lubricants, 1994, p. 26).
Meanwhile, although the quantity of the film is smaller in calcium-type phosphatization, the obtained crystals are fine and are highly workable, and therefore this type of phosphatization is used to prevent die clogging, which is a critical problem in forging and cold pressing. There are cases in which the excess phosphate film falls off during cold forging or cold pressing and adheres to the work piece, resulting in a fatal product defectxe2x80x94the inability to achieve the specified degree of precision with respect to dimensions.
Treatment with a reactive soap lubricant is achieved by immersing the phosphatized material in an aqueous solution of sodium or potassium soap (salts of fatty acids) heated to 60 to 95xc2x0 C. The reaction mechanism is that the phosphate film dissolves in the aqueous solution of soap, followed by the replacement reaction with at least some of the constituent cations comprising the phosphate crystals to produce a metallic soap that is much less soluble in water than the sodium or potassium soap dissolved in the aqueous treatment solution, and the precipitation of this less soluble metallic soap onto the phosphate film.
For example, the reaction between zinc phosphate tetrahydrate (Zn3(PO4)2.4H2O) and sodium stearate (C17H35COONa) produces zinc stearate, as shown in the following equation:
Zn3(PO4).4H2O+6C17H35COONaxe2x86x923(C17H35COO)2Zn+2Na3PO4+4H2O.
However, for reaction with aqueous solutions of soluble soaps, zinc phosphate has the fastest reaction rate, followed by zinc ferrous phosphate and zinc calcium phosphate in that order. Because zinc calcium phosphate has a particularly slow reaction rate, lubrication rarely involves combination with a reactive soap lubricant when the primary constituent of the phosphate film is zinc calcium phosphate.
Japanese Laid-Open Patent Application Sho[wa] 60-20463 discloses a cold processing lubrication method for iron and steel in which the surface of the iron and steel is first treated with a phosphate-forming treatment solution that contains calcium ions, zinc ions, phosphate ions, and nitrate ions, in which the weight ratio of the calcium ions to the zinc ions is 0.1 to 1.0, and the weight ratio of the nitrate ions to the phosphate ions is 1.0 to 5.0, and the pretreated material is then treated with a lubricant. This teaching is a treatment method that is ideal when using a reactive soap lubricant after calcium type phosphatization, and its primary purpose is to ensure that there are appropriate quantities of zinc calcium phosphate and zinc phosphate in the film by ensuring an appropriate weight ratio of calcium ions to zinc ions in the phosphatizing solution.
In addition, Japanese Laid-Open Patent Application Hei[sei] 8-215287 discloses a method of phosphatizing high carbon, chrome bearing steel wire in which the surface of the steel wire is treated with a phosphauizing solution that contains calcium ions, zinc ions, phosphate ions, and nitrate ions, in which the weight ratio of the calcium ions to the zinc ions is 0.5 to 1.5. This teaching pertains to a surface treatment method in which the high carbon, chrome bearing steel wire is treated with a lubricant whose primary constituents are calcium hydroxide, also called lime soap, and calcium soap, after undergoing calcium type phosphatization, with a primary purpose of ensuring the formation of a zinc calcium phosphate film that is protective against die clogging by ensuring an appropriate weight ratio of calcium ions to zinc ions in the phosphatizing solution.
However, when employing the methods described in the aforementioned Japanese Laid-Open Patent Application Sho[wa] 60-20463 and Japanese Laid-Open Patent Application Hei[sei] 8-215287, it was necessary to adjust the ratio of the calcium ions to the zinc ions in the phosphatizing solution within the specified range in order to obtain a phosphate film with the most preferred composition. In actual commercial operations, substrates with more than one processing purpose are treated in succession. Therefore, the ratio of calcium ions to zinc ions in the phosphatizing solution needs to be adjusted, or multiple phosphatization vessels need to be installed, in order to obtain a phosphate film with the preferred composition when employing the aforementioned methods in practical use on substrates for more than one processing purpose.
Furthermore, in order to adjust the phosphatzing solution, it is necessary to remove part of the solution from the phosphatization vat and add an additive that contains zinc ions or calcium ions; this makes the work rather complicated. When multiple vats are installed, each vat must always be maintained at a fixed concentration and temperature; thus, this method is disadvantageous in terms of work space and energy cost.
As described above, there were problems associated with conventional methods: The work space became rather large, the energy cost increased, and the operational properties were compromised.
The aim of this invention is to overcome at least one of the above mentioned problems associated with the prior art, and to provide a method of phosphatizing iron and steel by forming a phosphate film with the target composition on the surface of the iron and steel, thus resolving the problems of poor lubrication and die clogging.
It has been discovered that these problems could be resolved by treating iron and steel with a calcium phosphate-type phosphatizing solution after pre-treating the surface of the iron and steel with a surface conditioning solution with a specific composition, thereby perfecting this invention. In other words, this invention provides a method of phosphatizing iron and steel that, in one embodiment, is characterized by the fact that the surface of the iron and steel is first treated with a surface conditioning liquid composition that contains as its essential ingredient dispersed fine particles of at least one of the following water-insoluble compounds: zinc phosphate, zinc calcium phosphate, and calcium phosphate, and is then treated with a phosphatizing solution that contains calcium ions as its essential ingredient. In another embodiment, this invention provides a method of phosphatizing iron and steel that is characterized by the fact that the surface of the iron and steel is first treated with a surface conditioning liquid composition that contains colloidal titanium as its essential ingredient, and is then treated with a phosphatizing solution that contains calcium ions as its essential ingredient.